1. Field of the Invention
The present invention relates to an oil-ring for disposition in an oil-ring groove of a piston of an internal combustion engine. More specifically, the invention relates to a three-piece oil-ring consisting of a pair of side rails and an expander provided between the paired side rails.
2. Description of the Prior Art
Three-piece oil-rings have been known including a pair of side rails separated by an expander extending in a circumferential direction with waves in a radial direction. Both ends of the expander are opposed to each other. In this expander, side rail engaging portions of the expander are formed on both sides, axially, of radially inner portions of the expander so as to protrude in an axial direciton. The side rail engaging portions are engaged with inside surfaces of the pair of side rails, thereby transmitting the expanding spring force of the expander to the paired side rails, pushing the paired side rails slidably against a cylinder bore surface.
In the composite oil-ring including the above-mentioned radially waving expander, for the purpose of contacting the paired side rails to the cylinder bore surface during reciprocal motion of a piston, the side rail engaging portions of the expander must move along the cylinder bore surface in a radial direction. However, since the expander itself has rigidity, the side rail engaging portions formed on opposite sides of flat portions at the radially inner portions of the expander cause each other to deform. Also, typical cylinder bores do not have perfectly straight axial walls, thereby causing a piston slap during the receiprocal motion thereof. Therefore, one of the paired side rails may contact the cylinder bore surface while the other one does not. The result is an increase in oil consumption.
Since this problem arises due to the fact that the side rail engaging portion at one side of the expander is deformed by the opposite side rail engaging portion, various inventions to reduce rigidity or increase flexiblity of the expander have been proposed. For example, Japanese Utility Model Publication No. SHO 57-38956 disclosed a composite oil-ring, wherein spring arms extend from both upper and lower portions of an expander, tip portions of the arms contact an inside surface of a rail ring and a center portion of the rail ring is supported by a waving portion of the expander.
In the above composite oil-ring, since a rail engaging portion has very low rigidity, the rail engaging portion at one side of the expander receives little deforming influence from the opposite rail engaging portion. However, since the rail engaging portion extends for a relatively long distance, it is difficult to form the rail engaing portion in a planned shape. Also, if the rigidity of the rail engaging portion is too low, fluttering due to a resonant phenomenon may happen at high engine speeds, causing an increase in oil consumption.
Another Japanese Utility Model Publication No. SHO 59-107952 discloses a composite oil-ring, wherein a pair of side rails themselves are formed with low rigidity. However, by only weakening the rigidity of the paired side rails, reliable following of the paired side rails along a cylinder bore surface in a case of large fluctuation in a clearance between a piston and the cylinder bore surface may not be obtained.